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SCXI-1102 negative rail

Help,

 

I am using the SCXI-1102/ terminal block 1303/thermocouples to measure temperature of a device.  I have the thermocouples at different locations inside and outside of the device.  Everything works right, except for those thermocouple channels which are attached to the device under test (DUT) metal chassis.  They all read a high negative temperature which lead me to believe that they were at the negative rail.  Removing the thermocouple channel from the chassis brings the reading back to normal.

 

I measured the between the DUT chassis and the power supply ground - it read 0V.  I removed power from the DUT chassis and the problem didn't go away.  I also tried to bring the SCXI chassis and the chassis of the device under test to the same potential out of desperation but it didn't cure a problem.  It reads 9 mV between the screw terminals of the affected channels.  My only theory now is that it has something to do with the bias resistor setting but I am not sure what to do next.  Please help.  Thanks.

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You may have a ground loop.  If some current flows between the DUT chassis and the SXCI ground, it may produce voltages much larger than the normal thermocouple voltages and give youre the results you see.

 

For applications like this isolated thermocouples may be the best choice.

 

Lynn

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